Electric arc stabilization in electric arc melting using carbon electrodes

ABSTRACT

Arc furnace electrodes consist of an outer shell of carbon and an inner core consisting of a carbonaceous filler, a promoter material, e.g., an alkali metal oxide, carbonate or halide, and a carbonaceous binder such as pitch or tar.

United States Patent 1 1 1111 3,715,440 Robiette 1451 Feb. 6, 1973 154] ELECTRIC ARC STABILIZATION IN ELECTRIC ARC MELTING USING "[56] References Cited 7 CARBON ELECTRODES UNITED STATES PATENTS 5] Inventor: Alfred Gordon Evans Robiette,

1,444,961 2/1923 Eynon ..13/18 Necheus Brmmgham England 2,185,679 1/1940 Parisot [73] Assignee: Foseco International Limited 2,640,135 1953 Cobine 2,744,945 5/1956 Johnson Flledr April 1971 3,198,932 8/1965 Weatherly [21] pp No; 130,431 3,214,153 10/1965 Houffet al ..13/9 X 1 R l t d US A li i Data Primary Examiner-Roy N. Envall, Jr. Att0mey-W01fe, Hubbard, Leydig, Voit & Osann, [60] Divlslon of Ser. No. 791,505, Jan. 5, 1969, aban- Ltd.

doned, which is a continuation-in-part of Ser. No. 573,867, Aug. 23, 1966, Pat. No. 3,461,339. ABSTRACT [30] Foreign Application Priority Data Arc furnace electrodes consist of an outer shell of carbon and an inner core consisting of a carbonaceous 0C1.- 1, Great 81113111 n a promoter material, g an metal i 52 U.S. c1 ..13/1s, 313 354 3P and a carbonaceous bmder Such [51] Int. Cl. ..H05b 7/06 I 1 14 Claims, 1 Drawing Figure Field of Search ..13/9, 18; 313/354 ELECTRIC ARC STABILIZATION IN ELECTRIC ARC MELTING USING CARBON ELECTRODES CROSS REFERENCE TO RELATED APPLICATION This application is a division of my copending application Ser. No. 791,505, filed Jan. 5, 1969, now abandoned which in turn is a continuation in part of my application Ser. No. 573,867, filed Aug. 23, 1966, now U.S. Pat. No. 3,461,339.

DESCRIPTION OF THE INVENTION The present invention relates to carbon electrodes for are furnaces. It is an improvement in the invention described in U.S. Pat. applications Ser. No. 573,867.

In the electric arc furnace, an arc is generally used to melt a charge of scrap metal or to melt a charge of metal containing ore and reducing agent, for example a metal oxide and crushed coke. The arc may be struck directly between two electrodes, or more usually may be struck between the electrodes and the charge. For larger furnaces, e.g., over 10 tons capacity, the latter is almost always the case. The arcs used are generally of very high current, usually greater than 30,000 amps.

When the arc is struck between electrodes and the charge, the electrodes are subject to very severe heating, vibration and erosion. In addition, the striking and maintenance of an arc is extremely difficult, especially when the charge is of highly irregular shape, e.g., a charge of unmelted scrap metal. This is particularly the case during the initial stage ofmelting.

A further difficulty which arises in the arc furnaces is that of maintaining the arc when, as is usual, it is powered by an alternating current. In such a case, the voltage applied between the electrodes or between electrode and charge, momentarily passes through a zero value 100 times per second, and the are thus tends to break. The re-establishment of the arc in the subsequent half-cycle is difficult, especially shortly after operation has begun, while the charge is still of irregular shape. Indeed, the nature of a carbon are of high current density, as typified by those occurring in large industrial arc furnaces, is fundamentally different from those concerned with straight carbon-to-carbon arcs, as for instance in an arc lamp. The stability is of great importance in the former field for the sake of power supply and the danger of exceeding the maximum allowable surge whereas, with an arc lamp it is of little significance except as regards the flickering of the light produced. In the case of an arc furnace problems occur with such massive currents when passing through the zero point of an AC cycle; with a direct are between two carbon electrodes in a lamp, there is no problem of re-establishing the arc after the zero point. Additionally, a carbon-to-carbon arc is stable by nature as the distance between electrodes is not likely to vary and is in any case of comparatively small dimensions; on the other hand, the are between an electrode and a scrape charge is likely to suffer in that the distance between the arc and the charge is likely to vary and major fluctuations will occur as the charge melts and moves. The problems of stabilizing the arc are thus much greater in the latter case. In addition, the major energy transfer in an arc furnace is believed to be in the form of a plasma rather than as a cloud of ions or electrons as in the case ofa lamp.

It is accordingly an object of the present invention to provide carbon furnace electrodes which, in use, avoid the disadvantages set forth above, but which meet the necessary requirements ofsuch electrodes.

In the drawing the single FIGURE is a side elevation, partially in section, of an arc furnace electrode according to the present invention.

According to the present invention there are provided arc furnace carbon electrodes which consist of an outer hollow cylinder consisting substantially of carbon and an inner core, the said core comprising a major proportion of a carbonaceous filler, from 1 to 25 percent by weight of a material which under the action of the electric arc, in use, ionises to increase the electrical conductivity of the arc, and a carbonaceous binder. The material which, under the action of the arc, ionises to increase the electrical conductivity thereof is, for simplicity, hereinafter referred to simply as a promoter material.

The electrodes normally range in diameter from 4 inches to 24 inches. The diameter of the core is preferably from A to /1 the diameter of the electrode, respectively as the diameter of the original electrode is large or small.

The carbon electrode outer hollow cylinder is generally of substantially pure carbon and may be made by any of the methods known per se for making arc furnace carbon electrodes. The electrode may be made in the form of a solid cylinder which is subsequently bored, or directly in the form of a hollow cylinder.

The carbonaceous filler material, which is preferably present in the core in an amount of from 55 percent by weight, may be crushed coke, crushed calcined anthracite, crushed graphite or other similar material.

The promoter material, most preferably constitutes about 20 percent by weight of the core. Most preferably the promoter material is a compound of an alkali metal (preferably sodium or potassium for reasons of economy), for example a carbonate, oxide or halide. Of the halides, the iodides are preferred. Chromates, silicates, aluminates and aluminosilicates of the alkali metals are also usuable. The most preferred promoter materials are sodium carbonate, potassium carbonate, sodium iodide and potassium iodide. One or more promoter materials may be used in the core at the same time.

The binding agent in the core must, as noted above, be a carbonaceous binder; this is in order to ensure that the core is of sufficient electrical conductivity. Suitable carbonaceous binders are certain tars, pitches and bitumens. It is important to ensure that the relative coeff-v cients of thermal expansion of the core and the surrounding carbon electrode shell are not such that on heating the shell is cracked open or the core allowed to fall out. For this reason it is preferable to use a core which has some slight plasticity, such as may be obtained using a core bonded with a suitable tar, pitch or bitumen.

A suitable specification for the core binder (and one Water content: 0.5 percent by weight maximum Oils to 270C: 4 percent by weight maximum Oils to 300C: 8 percent by weight maximum The binder should also conform to the specifications laid down in 8.8.8. 1310 (1950).

inorder to securely anchor the core in the electrode it is preferable to provide the inner surface of the outer hollow cylinder with grooves, depressions or the like in which the core may engage.

The preferred method of making the cored electrodes is as follows:

The ingredients of the core in particulate form are dry mixed at room temperature to give a homogeneous mixture. This mixture is then heated to about 80C. to give a uniform thick mastic-like mixture. The hollow outer shell is heated to 60C. and the mixture rammed into the outer shell. Thereafter, the electrode may be baked for about 1 hour at 400C. to drive off low temperature volatile constiuents, although with larger electrodes this has been found unnecessary.

The following example will serve to illustrate the invention:

EXAMPLE calcined anthracite 60% pitch (softening point 50C. and

conforming to the specification given above) 20% 20% sodium carbonate This electrode was inserted into an 8 ton capacity 3- phase electric melting furnace, which was charged with steel scrap. The scrap charge was then melted and the performance of the cored electrode compared with .that of two similar standard electrodes used on the other two phases. The cored electrode showed no tendency to break up or erode more than the uncored electrode.

During the first to 10 minutes of melting, the cored electrode was arcing for a greater time than the uncored electrodes (50 to 200 percent more). After a time, continuous arcing was obtained on all three elec trodes. The cored electrode had a lower restriking voltage onto the metal charge, and the current zero pauses were much diminished. The strain on the transformer used to supply the furnace was accordingly slightly less (and, of course, would be much less if three cored electrodes were used).

Referring'to FIG. 1, there is shown an electrode comprising a hollow outer cylinder 11 of carbon, and an inner core 12 of a carbonaceous filler, a carbonaceous binder material and a suitable promoter material. The

core 12 is anchored to the outer cylinder 11 by means of grooves 13 formed in the core 12 and receiving complementary ribs formed on the inner surface of the cylinder 11.

lclaim:

1. A method for the stabilization of an electric arc in the melting of metal and in the production of metal from metal-containing ore and a reducing agent in an arc furnace, said method comprising the steps of forming an electrode comprising an outer hollow cylinder consisting substantial y of carbon, and an inner core,

the said core comprising a major proportion of a carbonaceous filler, from I to 25 percent by weight of a promoter material selected from the group consisting of alkali metal halides, chromates, aluminates and alumino silicates, and a carbonaceous binder; inserting said electrode in an arc furnace containing metal or metal-containing ore and a reducing agent; and ionizing said promoter material by forming an arc between said metal and said electrode whereby the ionization of said promoter material increases the electrical conductivity of the arc.

2. A method according to claim 1 wherein said electrode has a diameter of from 4 inches to 24 inches.

3. A method according to claim 1 wherein the diameter of the core is V: to V; the diameter of the electrode.

4. A method according to claim 1 wherein the outer hollow cylinder consists of substantially pure carbon.

5. A method according to claim 1 wherein the carbonaceous filler is present in a proportion of from 55-90 percent by weight of the core.

6. A method according to claim 1 wherein the carbonaceous filler is selected from the class consisting of crushed calcined anthracite and crushed graphite.

7. A method according to claim 1 wherein the promoter material constitutes about 20 percent by weight of the core.

8. A method according to claim 1 wherein the promoter material is sodium carbonate.

9. A method according to claim 1 wherein the promoter material is potassium carbonate.

10. A method according to claim 1 wherein the promoter material is sodium iodide.

11. A method according to claim 1 wherein the promoter material is potassium iodide.

12. A method according to claim 1 wherein the carbonaceous binder is selected from the class consisting of tars, pitches and bitumens.

13. A method according to claim 1 wherein the hollow cylinder is provided on its inner surface with grooves serving to anchor the core in the hollow outer cylinder.

14. A method according to claim 1 wherein the ho]- low outer cylinder is provided on its inner surface with depressions serving to anchor the core in the hollow outer cylinder. 

1. A method for the stabilization of an electric arc in the melting of metal and in the production of metal from metal-containing ore and a reducing agent in an arc furnace, said method comprising the steps of forming an electrode comprising an outer hollow cylinder consisting substantially of carbon, and an inner core, the said core comprising a major proportion of a carbonaceous filler, from 1 to 25 percent by weight of a promoter material selected from the group consisting of alkali metal halides, chromates, aluminates and alumino silicates, and a carbonaceous binder; inserting said electrode in an arc furnace containing metal or metal-containing ore and a reducing agent; and ionizing said promoter material by forming an arc between said metal and said electrode whereby the ionization of said promoter material increases the electrical conductivity of the arc.
 2. A method according to claim 1 wherein said electrode has a diameter of from 4 inches to 24 inches.
 3. A method according to claim 1 wherein the diameter of the core is 1/4 to 1/2 the diameter of the electrode.
 4. A method according to claim 1 wherein the outer hollow cylinder consists of substantially pure carbon.
 5. A method according to claim 1 wherein the carbonaceous filler is present in a proportion of from 55- 90 percent by weight of the core.
 6. A method according to claim 1 wherein the caRbonaceous filler is selected from the class consisting of crushed calcined anthracite and crushed graphite.
 7. A method according to claim 1 wherein the promoter material constitutes about 20 percent by weight of the core.
 8. A method according to claim 1 wherein the promoter material is sodium carbonate.
 9. A method according to claim 1 wherein the promoter material is potassium carbonate.
 10. A method according to claim 1 wherein the promoter material is sodium iodide.
 11. A method according to claim 1 wherein the promoter material is potassium iodide.
 12. A method according to claim 1 wherein the carbonaceous binder is selected from the class consisting of tars, pitches and bitumens.
 13. A method according to claim 1 wherein the hollow cylinder is provided on its inner surface with grooves serving to anchor the core in the hollow outer cylinder. 